Altitudes Explained

A drone's height can be measured from several starting points, and Remote ID reports several of them. The exports keep each one under its own name, so you always know which one you are reading.

JSON type	CSV column	Measured from	Comes from
HAE-WGS84	alt_hae_wgs84	The WGS 84 ellipsoid, the smooth model of the Earth that GPS uses	The drone (GPS)
MSL	alt_msl	Mean sea level	The drone, or calculated by Dronetag
ATO	alt_ato	The take-off point	The drone
PA-QNE	alt_pressure	Standard air pressure (1013.25 hPa)	The drone (barometer)
AGL	alt_agl	The ground under the drone	Calculated by Dronetag

All values are in metres.

One Point, Five Altitudes

A point from the example detection, about 60 m above the ground:

Type	Value	What it means
HAE-WGS84	459.4 m	Height above the ellipsoid. In Czechia the ellipsoid lies about 45 m below sea level, so this number is the highest.
MSL	413.9 m	Altitude above sea level
ATO	60.9 m	Height above the place it took off from
AGL	60.0 m	Height above the ground under it. It is close to ATO here because the ground is about as high as at take-off. On a slope the two differ.
PA-QNE	388.6 m	The barometer's reading. It is 25 m off the sea-level altitude because real air pressure is never exactly standard.

Which One to Use

Question	Use
How high above the ground?	AGL
How high above where it took off?	ATO
How high above sea level, for example to compare with airspace limits?	MSL
What did the GPS say?	HAE-WGS84

Pressure altitude changes with the weather and isn't corrected for local air pressure. Don't use it as height above sea level.

Why the height above take-off can be negative, and why many drones show it as "AGL", is explained in Understanding Altitude Values.

When MSL Is Empty

If the drone reported a sea-level altitude, the export keeps it. Otherwise Dronetag calculates one from the GPS height. If there is no GPS height, Dronetag uses the height of the take-off point plus the height above take-off.

When none of this is possible, MSL is empty. The file doesn't say which way the value was found.

When AGL Is Empty

AGL is the sea-level altitude minus the height of the ground at that place. The ground height comes from a terrain model. AGL is empty when there is no sea-level altitude or no terrain data for that place. See Terrain Data and Attribution.

A height slightly below zero is normal for a drone standing on the ground. The example flight starts at 0.43 m and goes down to -1.04 m before take-off. Values this close to zero mean "on the ground".

Over forest and in towns, AGL can read low. Some terrain models measure the top of trees and roofs, not the soil.

Accuracy

In JSON, an altitude can have an accuracy in metres. Roughly two times in three, the true value is within this amount.

• A value the drone reported has an accuracy only if the drone reported one.
• AGL has an accuracy.
• A calculated MSL has one when the value it was calculated from had one. It also includes the error of the conversion, 0.4 m through the geoid or 3.5 m through the take-off point.

CSV files have no accuracies. KMZ and PDF show the height above ground as an estimate, for example "97 m (estimated, ±3.0 m)".

KMZ and PDF calculate the height above ground from the GPS height. When a drone also reported its own sea-level altitude, their figure can differ a little from the one in JSON and CSV.

Altitudes in the Summary

The summary of each flight or detection has the highest value of each altitude type. In JSON this is max_altitudes. In CSV Summary it is max_alt_ato, max_alt_msl and max_alt_agl. There are no minimums.